In the case of forming a color image by using an ink composition, at least a yellow ink, a magenta ink and a cyan ink are generally used. There can be reproduced a color in the green region when mixing yellow and cyan colors, a color in the red region when mixing yellow and magenta colors, and a color in the blue region when mixing cyan and magenta colors. Accordingly, development of an ink set capable of successfully reproducing colors in these regions over a wide range has been long demanded.
Each ink must satisfy performances generally required of the ink composition. For example, it is demanded that a sharp image is obtained on a recording medium and the image is free from deterioration during storage for a long time. In particular, one of performances required of an image formed using the ink composition is light fastness.
In order to impart good light fastness, a pigment is generally used as the colorant in the ink composition. However, depending on the pigment selected, sufficiently high light fastness may not be imparted. Also, the pigment is inferior in the coloring power compared with a dye and moreover, the image obtained by printing on a recording medium may lack the sharpness.
In the case of a color image formed using plural ink compositions, if any color inferior in the light fastness is present, the hue of the image changes and the quality of the color image seriously deteriorates. Accordingly, more controlled light fastness is required of a color ink composition.
In recent years, an inkjet recording printer is widely spreading. The inkjet recording method is a printing method of flying an ink composition in the form of small droplets and attaching it onto a recording medium such as paper, thereby performing printing. This method is characterized in that a high-resolution and high-quality image can be printed at a high speed by a relatively inexpensive apparatus. In particular, thanks to the enhanced image quality, a color inkjet recording apparatus is used also as a photographic output machine and is utilized even as a digital printer, a plotter, a CAD output device or the like. In using the image printed by the inkjet recording printer, various forms may be considered. Above all, a photograph-grade print is possibly put as a display in a place where the print is exposed to fluorescent light or direct sunshine, for example, in outdoor conditions for a long period of time. Accordingly, light fastness is one of very important performances required of the image obtained by the inkjet recording method.
As regards the pigment-based ink set excellent in the color reproducibility and light fastness over a wide range, particularly, as for the yellow pigment ink satisfying both good hue and high light fastness, an ink satisfying the required performances in a high level is not present, and development of a yellow pigment ink has been long demanded.
At the formation of a color image, a yellow ink composition containing a yellow pigment is used for reproducing a color in the yellow region. Specific examples of the yellow pigment include C.I. (Colour Index International) Pigment Yellow 1, 2, 3, 12, 13, 14, 16, 17, 73, 74, 75, 83, 93, 95, 97, 98, 109, 110, 114, 120, 128, 129, 138, 139, 150, 151, 154, 155, 180, 185 and 213.
Also, it is disclosed in JP-A-2005-314545 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and Japanese Patent No. 3,911,920 that a plurality of yellow pigment compositions each combining the above-described yellow pigments are prepared so as to reproduce a color in the yellow region over a wide range.
The yellow ink composition with good color reproducibility containing two or more kinds of yellow pigments may realize color reproducibility in a wider range than is possible by an ink containing one kind of a pigment, but color reproducibility of the dark part in the yellow region is not so high. Also, mixing of pigment species having hue angles distant from each other has a prominent tendency to cause reduction in the secondary color saturation. For example, C.I. Pigment Yellow 110 is red-tinged and its use in an ink set has a drawback that the green color shows a low saturation.
Furthermore, it is required not only that a sharp image is obtained on a recording medium but also that the image does not deteriorate during storage for a long time (particularly, light fastness). In the case of forming a color image by using plural pigment ink compositions, if any color inferior in the light fastness is present, the hue of the image changes and the quality of the color image extremely deteriorates. For example, C.I. Pigment Yellow 74 has a relatively good hue but because of its extremely low light fastness, when used in an ink set, the hue of the image greatly changes in the yellow, red, green and gray parts, and the quality deterioration of a color image is a major problem to be solved.
On the other hand, in order to reproduce a color in the yellow region over a wide range, Japanese Patent No. 3,455,764 Japanese Patent No. 3,553,581 and Japanese Patent No. 3,882,418 disclose a method using an ink set where a low-brightness yellow ink called dark yellow is further equipped as a second ink composition, separately from a high-brightness yellow ink.
However, in the method using a second ink composition typified by dark yellow, the number of ink colors is increased to make the color preparation complicated and not only the cost tends to rise but also the improvement level fails in reaching the level sufficiently satisfying the required performances.
Furthermore, for the purpose of imparting light fastness and coloring power (saturation) to the yellow ink composition, JP-A-2008-266568 discloses a method using an ink set where one or more pigments selected from C.I. Pigment Yellow 155, 185 and 213 are contained as the yellow pigment.
However, this method does not reach the level sufficiently satisfying good hue, high light fastness and high coloring power.